Effects of bull, sperm type and sperm pretreatment on male pronuclear formation after intracytoplasmic sperm injection in cattle.

This study investigated the effects of the bull, sperm type (dead, immotile or motile) and sperm pretreatment (i.e. mechanical (tail-cutting or tail-scoring) or chemical (heparin, heparin + caffeine, calcium ionophore A23187 or dithiothreitol)) on male pronuclear formation after intracytoplasmic sperm injection (ICSI) in cattle. Three experiments were conducted. In Experiment 1, spermatozoa from three bulls (A, B and C) were used for both ICSI and in vitro fertilization (IVF). The results were that sperm from bull B yielded a higher penetration/male pronuclear formation rate than that of bull C when used for IVF (89.6% v 25.6%, P<0.01). However, when injected into oocytes by ICSI, sperm from bull C had a higher male pronuclear formation rate than that of bull B (34.6% v. 16.1%, P<0.05). The effects of sperm type and mechanical pretreatment were examined in Experiment 2. No significant difference was found in the male pronuclear formation rate when the three types of sperm were injected into oocytes. Tail-scored sperm achieved a higher male pronuclear rate than that of non-mechanically treated ones (38.2% v. 13.2%, P<0.005). In Experiment 3, chemical pretreatments were tested and compared. Higher male pronuclear rates, compared with the control, were obtained when sperm were pretreated with heparin + caffeine, calcium ionophore A23187 and dithiothreitol (48.2%, 62.5% and 64.5% v. 25.0%, P<0.05, 0.005 and 0.005, respectively). These results indicate that (1) there is a bull variation in male pronuclear formation with ICSI, and the male pronuclear rate by ICSI is not coincident with the results by IVF, (2) immobilization of a spermatozoon by tail-scoring before ICSI can improve the formation of the male pronucleus, and (3) an appropriate chemical pretreatment of spermatozoa is necessary to achieve a higher rate of male pronuclear formation.